Valgrind detects memory leak despite the fact memory has been freed

Question

I have a file "a", with 2000 characters, char "a" only, no spaces.

Then I have this code, that runs trough the loop, add it to buffer, eventually reallocs if the limit is reached and on errors it frees strBuffer variable.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main()
{
    int maxS = 200;
    int numericExpression;
    int strLength;


    char *strBuffer;
    strBuffer = malloc(sizeof(char)*maxS+1);
        if(strBuffer == NULL)
        {
            printf("Failed to allocate requested memory!\n");
            free(strBuffer);
            strLength = sizeof(strBuffer);
            printf("Freed %d bytes of memory!\n", strLength);
            exit(99);
        }
        else
        {
            numericExpression = sizeof(char)*maxS+1;
            printf("Alocated: %d Bytes of memory.\n", numericExpression);
        }

    // while simulation

    int fcv = -1;
    int numEx;


    // file opening simulation
    FILE* fd = fopen("a", "r");


    int c;
    while((c=fgetc(fd) != EOF)) // condition to make sure we realloc only once
    {
        fcv++;
        strBuffer[fcv] = c;
        if(fcv == (maxS))   
        {
            printf("Additional memory space required!\n");      
            int strlensize = strlen(strBuffer); 
            numEx = (sizeof(char)*(2*strlensize));


            strBuffer = realloc(strBuffer, numEx);

            if(strBuffer == NULL)
            {
                printf("Failed to allocate requested memory!\n");
                strLength = sizeof(strBuffer);
                free(strBuffer);
                printf("Freed %d bytes of memory!\n", strLength);
                exit(99);
            }
            else
            {
                maxS = numEx;
                printf("Reallocation successful!\n");
                printf("Alocated: %d Bytes of memory.\n", numEx);
            }

        }

    }
    strLength = sizeof(strBuffer);
    free(strBuffer);
    printf("Freed %d bytes of memory!\n", strLength);
}

Problem is that it tells me in the end, that I freed only 8 bytes of memory. I suppose this is because sizeof(strBuffer) doesn't respond to the expected size. When I use strlen(strBuffer) instead, I free only 2001 bytes.

I suppose this could be only problem of printing out the amount of bytes freed. I could be doing it wrong. So maybe I'm just not able to tell how many bytes I free. But then I try valgrind and it tells me, I don't free enough, that there are memory leaks. But in every branch of a program, I get to free the memory used by strBuffer.

When I run it through valgrind ("valgrind ./realloc"), it tells me this:

==780== Memcheck, a memory error detector
==780== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==780== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==780== Command: ./realloc
==780== 
Alocated: 201 Bytes of memory.
Additional memory space required!
==780== Invalid read of size 1
==780==    at 0x4C2E0F4: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400846: main (in /home/dan/Desktop/test_ifj/realloc)
==780==  Address 0x51fd109 is 0 bytes after a block of size 201 alloc'd
==780==    at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x40078C: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Reallocation successful!
Alocated: 402 Bytes of memory.
==780== Invalid write of size 1
==780==    at 0x400823: main (in /home/dan/Desktop/test_ifj/realloc)
==780==  Address 0x51fd562 is 0 bytes after a block of size 402 alloc'd
==780==    at 0x4C2CE8E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400866: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Additional memory space required!
Reallocation successful!
Alocated: 806 Bytes of memory.
Additional memory space required!
==780== Conditional jump or move depends on uninitialised value(s)
==780==    at 0x4C2E0F8: strlen (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==780==    by 0x400846: main (in /home/dan/Desktop/test_ifj/realloc)
==780== 
Reallocation successful!
Alocated: 804 Bytes of memory.
Freed 8 bytes of memory!
==780== 
==780== HEAP SUMMARY:
==780==     in use at exit: 568 bytes in 1 blocks
==780==   total heap usage: 5 allocs, 4 frees, 2,781 bytes allocated
==780== 
==780== LEAK SUMMARY:
==780==    definitely lost: 0 bytes in 0 blocks
==780==    indirectly lost: 0 bytes in 0 blocks
==780==      possibly lost: 0 bytes in 0 blocks
==780==    still reachable: 568 bytes in 1 blocks
==780==         suppressed: 0 bytes in 0 blocks
==780== Rerun with --leak-check=full to see details of leaked memory
==780== 
==780== For counts of detected and suppressed errors, rerun with: -v
==780== Use --track-origins=yes to see where uninitialised values come from
==780== ERROR SUMMARY: 1200 errors from 3 contexts (suppressed: 0 from 0)

How do I properly free memory I have allocated? So it won't cause memory leaks? Eventually - am I doing it wrong and there's a better way to do it? Thank you for your help.

QUESTION UPDATE

I followed advises and got it to 3 errors in 1 context. This is how my code looks now:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main()
{
    int maxS = 200;
    int numericExpression;


    char *strBuffer;
    strBuffer = malloc((maxS+1));
        if(strBuffer == NULL)
        {
            printf("Failed to allocate requested memory!\n");
            printf("Freed %d bytes of memory!\n", maxS);
            exit(99);
        }
        else
        {
            numericExpression = sizeof(char)*maxS+1;
            printf("Alocated: %d Bytes of memory.\n", numericExpression);
        }

    // while simulation

    int fcv = -1;
    int numEx;


    // file opening simulation
    FILE* fd = fopen("a", "r");

    if(fd == NULL)
    {
        printf("Error opening a file!\n");

        if(strBuffer != NULL)
        {free(strBuffer);}


        exit(99);
    }


    int c;

    char *tmpBuffer;
    while((c=fgetc(fd)) != EOF) // condition to make sure we realloc only once
    {
        fcv++;
        strBuffer[fcv] = c;

        if(fcv == (maxS))   
        {
            printf("Additional memory space required!\n");      

            numEx = ((2*fcv));


            tmpBuffer = realloc(strBuffer, numEx);
            if(!tmpBuffer)
            {
                free(strBuffer);
                printf("Realloc() failed!\n");
                exit(99);
            }       
            else
            {
                strBuffer = tmpBuffer;
            }   


            if(strBuffer == NULL)
            {
                printf("Failed to allocate requested memory!\n");
                printf("Freed %d bytes of memory!\n", maxS); // well this is questionable, I think
                exit(99);
            }
            else
            {
                maxS = numEx;
                printf("Reallocation successful!\n");
                printf("Alocated: %d Bytes of memory.\n", maxS);
            }

        }

    }

    free(strBuffer);fclose(fd); // ADDED, still errors occur

    printf("Freed %d bytes of memory!\n", maxS);
}

And with the same valgrind call ("valgrind ./realloc") I get this:

==1213== Invalid write of size 1
==1213==    at 0x4007FD: main (in /home/dan/Desktop/test_ifj/realloc)
==1213==  Address 0x51fd560 is 0 bytes after a block of size 400 alloc'd
==1213==    at 0x4C2CE8E: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==1213==    by 0x400831: main (in /home/dan/Desktop/test_ifj/realloc)
==1213== 
Additional memory space required!
Reallocation successful!
Alocated: 800 Bytes of memory.
Additional memory space required!
Reallocation successful!
Alocated: 1600 Bytes of memory.
Additional memory space required!
Reallocation successful!
Alocated: 3200 Bytes of memory.
Freed 3200 bytes of memory!
==1213== 
==1213== HEAP SUMMARY:
==1213==     in use at exit: 568 bytes in 1 blocks
==1213==   total heap usage: 6 allocs, 5 frees, 6,769 bytes allocated
==1213== 
==1213== LEAK SUMMARY:
==1213==    definitely lost: 0 bytes in 0 blocks
==1213==    indirectly lost: 0 bytes in 0 blocks
==1213==      possibly lost: 0 bytes in 0 blocks
==1213==    still reachable: 568 bytes in 1 blocks
==1213==         suppressed: 0 bytes in 0 blocks
==1213== Rerun with --leak-check=full to see details of leaked memory
==1213== 
==1213== For counts of detected and suppressed errors, rerun with: -v
==1213== ERROR SUMMARY: 3 errors from 1 contexts (suppressed: 0 from 0)

Any tips what could cause this one?

Answer 1

this compiles, and does the job
it includes error handling
it eliminated many meaningless variables
it eliminates the errors in the logic of the OPs code



#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main()
{
    int maxS = 200; // current allocation size

    char *strBuffer = NULL;
    if( NULL == (strBuffer = malloc(maxS) ) )
    { // then, malloc failed
        perror( "malloc failed" );
        exit(99);
    }

    // implied else, malloc successful

    printf("Alocated: %d Bytes of memory.\n", )maxS+1));

    // file opening simulation
    FILE* fd = fopen("a", "r");
    if(fd == NULL)
    { // then fopen failed
        perror( "fopen failed for file: a" );
        free(strBuffer);
        exit(99);
    }

    // implied else, fopen successful

    int c;       // receives input char from fgetc()
    int fcv = 0; // index into malloc'd memory

    // tmpBuffer used in realloc()
    // so will not lose pointer to already allocated memory
    // in case realloc() fails
    char *tmpBuffer;

    while((c=fgetc(fd)) != EOF)
    {
        strBuffer[fcv] = c;
        fcv++;

        if(fcv >= maxS)
        {
            printf("Additional memory space required!\n");

            if( NULL == ()tmpBuffer = realloc(strBuffer, 2*maxS) )
            {
                perror( "realloc failed" );
                free(strBuffer);
                fclose(fd);
                exit(99);
            }

            // implied else, realloc successful

            maxS *= 2;  // only update after being sure realloc successful
            strBuffer = tmpBuffer;

            printf("Reallocation successful!\n");
            printf("Allocated: %d Bytes of memory.\n", maxS);
        } // end if
    } // end while

    free(strBuffer);
    fclose(fd);

    printf("Freed %d bytes of memory!\n", maxS);
    return( 0 );
} // end function: main

Answer 2

Well, everybody up to now gave you really important advises and you should consider using them (mainly the tempBuffer). BUT your problem is that you forgot to close your file descriptor:

fclose(fd);

Also, sizeof is compile time, thus it cannot give you dynamic allocated memory size and strlen needs a \n character to work. Calculating allocated and freed memory is a hard task and its solution it's not so trivial.

Answer Update

I executed your updated code and I only got 1 error from 1 context, which can be solved by changing the following line: tmpBuffer = realloc(strBuffer, numEx + 1);

Other than that, all memory, after fclose(fd), is free. I'm on a Ubuntu 12.04 machine using gcc 4.8.1.

Answer 3

This is your problem:

strBuffer = realloc(strBuffer, numEx);

If your call to realloc() fails, then it returns a null pointer, but does not free the original memory allocation.

You need to check the return value first, then assign it to the original pointer if successful:

char *tmpBuffer = realloc(strBuffer, numEx);
if (!tmpBuffer) {
  free(strBuffer);
  puts("realloc() failed");
  exit(1);
}
else {
  strBuffer = tmpBuffer;
}

---

There are a few other issues with your code, including:

• If strBuffer is NULL, then there is no point passing it to free().

• sizeof() is not a runtime function, so it has no idea how much memory was allocated.

• strBuffer = malloc(sizeof(char)*maxS+1); is a bit sloppy. I think you meant strBuffer = malloc(sizeof(char)*(maxS+1));, but you could just have put strBuffer = malloc(maxS+1); since sizeof(char) is 1 by definition.

Answer 4

while((c=fgetc(fd) != EOF)) should be while((c=fgetc(fd)) != EOF). The result of this is that you're trying to store 1 in your string instead of the character you read.

The memory problem stems from strlen(strBuffer);. You are calling strlen on something that is not a null-terminated string, which causes undefined behaviour. (This shows up as the valgrind report saying "Invalid read of size 1 in strlen).

To fix this, get rid of strlenSize and do:

maxS = 2 * maxS;
strBuffer = realloc(strBuffer, maxS + 1);

Note that if you want a "clean" valgrind in the case when you ran out of memory, you will need to check the return value of realloc before assigning it to strBuffer, as squeamish ossifrage points out.

---

NB. Your error handling code is poor. sizeof(strBuffer) finds the size of a pointer. The value you wanted to print is maxS. Also, free(NULL) has no effect; and there is no point having an else block after a block in which you called exit().